The focus of our research program is to understand the role of coaggregation in bacterial accretion of early colonizing bacteria on a clean tooth surface. The primary colonizers include actinomyces, streptococci, and veillonellae. Antiserum against the 38-kDa surface adhesin from S. gordonii PK488 cross reacts with a similar size protein from all of those streptococci (so far tested) that coaggregate with A. naeslundii PK606. The gene encoding this protein in S. gordonii PK488 has been cloned in a 2.1 kb DNA fragment and expressed in E. coli. Southern blots using the radioactively labeled 2.1 kb fragment as a probe identified a single restriction enzyme fragment in the genomic DNA of all streptococcal strains that coaggregate with A. naeslundii PK606. Identical fragments reacted with a 30-mer probe prepared from a gene encoding a 34.7-kDa saliva-binding adhesin of S. sanguis 12. This protein was shown to be a lipoprotein. These results suggest that a 34 to 38-kDa adhesin/lipoprotein may be present on most, if not all, early colonizing streptococci, and it may be important in mediating colonization of the tooth surface. The adhesins on several actinomyces including A. naeslundii PK606 are being investigated to determine the nature of functionally similar actinomyces adhesins. While intergeneric coaggregation among oral bacteria is commonplace, intrageneric coaggregation among oral bacteria is highly unusual, except among streptococci. Transposon mutagenesis has been used to identify a 110-kDa adhesin on S. gordonii DL1 that mediates intrageneric coaggregation with other streptococci. The gene encoding this adhesin is being cloned and sequenced. The long range goal of these studies, collectively, is to elucidate the molecular mechanisms responsible for bacterial colonization in the human oral ecosystem.